Effect of enhanced voluntary physical exercise on brain levels of monoamines in Huntington disease mice

A systematic review and meta-analysis of depression and apathy frequency in adult-onset Huntington's disease

Depression and apathy are associated with decreased functional capacity in Huntington's disease (HD) but frequency of depression and apathy in HD is largely unknown. Systematic literature searching was conducted across 21 databases until 30 June 2021. Inclusion criteria was limited to clinician-rated assessments of depression and apathy and adult-onset HD. Inverse-variance heterogeneity meta-analyses were conducted exploring depression and apathy frequency within individuals from families affected by HD, and within individuals with confirmed HD gene-positive status. Screening identified 289 articles for full-text review; nine remained for meta-analysis. Depression frequency in the lifetime in adults affected by or at-risk for HD was 38%, I² = 99%. Apathy frequency in the lifetime in adults affected by or at-risk for HD was 40%, I² = 96%. The robustness of the findings improved when limiting the analysis to gene-positive individuals only where apathy was found to be slightly more common than depression, 48% and 43% respectively. Future studies may consider reporting results from juvenile-onset HD and adult-onset HD cohorts separately to further explore phenotypic profiles.

Physical exercise prevents behavioral alterations in a reserpine-treated zebrafish: A putative depression model

Major depressive disorder (MDD) has increasingly reached the world population with an expressive increase in recent years due to the COVID-19 pandemic. Here we used adult zebrafish (Danio rerio) as a model to verify the effects of reserpine on behavior and neurotransmitter levels. We observed an increase in the immobile time and time spent in the bottom zone of the tank in reserpine-exposed animals. The results demonstrated a decrease in distance traveled and velocity. Reserpine exposure did not induce changes in memory and social interaction compared to the control group. We also evaluated the influence of exposure to fluoxetine, a well-known antidepressant, on the behavior of reserpine-exposed animals. We observed a reversal of behavioral alterations caused by reserpine. To verify whether behavioral alterations in the putative depression model induced by reserpine could be prevented, the animals were subjected to physical exercise for 6 weeks. The results showed a protective effect of the physical exercise against the behavioral changes caused by reserpine in zebrafish. In addition, we observed a reduction in dopamine and serotonin levels and an increase in the 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the brain. Physical exercise was able to prevent the changes in dopamine and serotonin levels, reinforcing that the preventive effect promoted by physical exercise is related to the modulation of neurotransmitter levels. Our findings showed that reserpine was effective in the induction of a putative depression model in zebrafish and that physical exercise may be an alternative to prevent the effects induced by reserpine.

The selective 5-HT 1A receptor agonist, NLX-112, overcomes tetrabenazine-induced catalepsy and depression-like behavior in the rat

Tetrabenazine, a preferential inhibitor of the vesicular monoamine transporter type 2, depletes the brain monoamines dopamine, serotonin and norepinephrine. Tetrabenazine and deutetrabenazine (Austedo ®) are used to treat chorea associated with Huntington's disease. However, both compounds are known to aggravate Parkinsonism and depression observed in Huntington's disease patients. NLX-112 (a.k.a. befiradol/F13640) is a highly selective, potent and efficacious serotonin 5-HT 1A agonist. In animal models, it has robust efficacy in combating other iatrogenic motor disorders such as L-DOPA-induced dyskinesia and has marked antidepressant-like activity in rodent tests. In the present study, we investigated, in rats, the efficacy of NLX-112 to counteract tetrabenazine-induced catalepsy (a model of Parkinsonism) and tetrabenazine-induced potentiation of immobility in the forced swim test (FST, a model to detect antidepressant-like activity). The prototypical 5-HT 1A agonist, (±)8-OH-DPAT, and the 5-HT 1A partial agonist/dopamine D2 receptor blocker, buspirone, were used as comparators. Both NLX-112 and (±)8-OH-DPAT (0.16-2.5 mg/kg p.o. or s.c., respectively) abolished catalepsy induced by tetrabenazine (2 mg/kg i.p.). In comparison, buspirone (0.63-5.0 mg/kg p.o.) was ineffective and even tended to potentiate tetrabenazine-induced catalepsy at 0.63 mg/kg. In the FST, NLX-112 and (±)8-OH-DPAT (0.63 mg/kg) strongly reduced immobility when administered alone but also significantly opposed potentiation of immobility induced by tetrabenazine (1.5 mg/kg i.p.). Buspirone (0.63 and 2.5 mg/kg p.o.) had no effect by itself or against tetrabenazine. These results strongly suggest that selective and highly efficacious 5-HT 1A agonists, such as NLX-112, may be useful in combating tetrabenazine-induced Parkinsonism and/or depression in Huntington's disease patients.

Bidirectional Association between Physical Activity and Dopamine Across Adulthood-A Systematic Review

Physical activity (PA) may influence the secretion of neurotransmitters and thereby have positive consequences for an individual’s vulnerability (i.e., reducing anxiety and depressive symptoms). This systematic review aims to analyse the potential bidirectional effects of exercise on dopamine from young adulthood to old age. The article search was conducted in PubMed, Scopus, and Web of Science in December 2020. The inclusion criteria were longitudinal and experimental study design; outcomes included dopamine and exercise; effect of exercise on dopamine and vice versa; adults; and articles published in English, Portuguese, or Spanish. Fifteen articles were included in the review. We observed robust findings concerning the potential effects of PA on dopamine, which notably seem to be observable across a wide range of participants characteristics (including age and sex), a variety of PA characteristics, and a broad set of methods to analyse dopamine. By contrast, regarding the potential effects of dopamine on PA, findings were mixed across studies. Thus, there are robust effects of physical exercise on dopamine. These findings further strengthen the idea that innovative approaches could include PA interventions for treating and preventing mental disorders. Therefore, it seems that PA is a potential alternative to deal with mental health issues.

The absence of the aryl hydrocarbon receptor in the R6/1 transgenic mouse model of Huntington's disease improves the neurological phenotype

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by an abnormal CAG repeat expansion in the huntingtin gene coding for a protein with an elongated polyglutamine sequence. HD patients present choreiform movements, which are caused by the loss of neurons in the striatum and cerebral cortex. Previous reports indicate that the absence of the aryl hydrocarbon receptor (AhR) protects mice from excitotoxic insults and increases the transcription of neurotrophic factors. Based on these data, we evaluated the effects of the lack of the AhR on a mice model of HD, generating a double transgenic mouse, expressing human mutated huntingtin (R6/1 mice) and knockout for the AhR. Our results show that the body weight of 30-week-old double transgenic mice is similar to that of R6/1 mice; however, feet clasping, an indicative of neuronal damage in the R6/1 animals, was not observed. In addition, motor coordination and ambulatory behavior in double transgenic mice did not deteriorate over time as occur in the R6/1 mice. Moreover, the anxiety behavior of double transgenic mice was similar to wild type mice. Interestingly, astrogliosis is also reduced in the double transgenic mice. The present data demonstrate that the complete loss of the AhR reduces the motor and behavioral deterioration observed in R6/1 mice, suggesting that the pharmacological modulation of the AhR could be a therapeutic target in HD.

c-Fos expression in the ascending arousal system induced by physical exercise in rats: Implication for memory performance

During exercise, multiple sensory information such as visual outflow, proprioception, and vestibular information promote an increase in arousal state, which may convey positive effects on cognitive abilities such as memory. Nevertheless, which of the components of the ascending arousal system (AAS) are engaged during physical activity and which of them are critical for cognitive enhancement, induced by exercise is still unclear. Two experiments were conducted, to answer these questions: in the first one, the neuronal activity of different components of the AAS was evaluated by c-Fos immunoreactivity (Fos-ir) in running rats exposed to a lock or unlock running wheel. We found a specific Fos-ir increase in the tuberomammillary nucleus (TMN) associated with physical exercise. In the second experiment sedentary and exercised rats were challenged to conduct an object recognition memory task, and the activity of the AAS after learning was evaluated by c-Fos immunoreactivity. The exercised group showed a higher performance in the object recognition memory task which gets correlated with an increase on Fos-ir in the TMN, but not with the other components of the AAS, suggesting that the increase on TMN activity induced by exercise may be the foremost contributor of the AAS to memory enhancement observed in exercised animals.

Short-Term Environmental Stimulation Spatiotemporally Modulates Specific Serotonin Receptor Gene Expression and Behavioral Pharmacology in a Sexually Dimorphic Manner in Huntington's Disease Transgenic Mice

Huntington’s disease (HD) is a neurodegenerative disorder caused by a tandem repeat mutation encoding an expanded polyglutamine tract in the huntingtin protein, which leads to cognitive, psychiatric and motor dysfunction. Exposure to environmental enrichment (EE), which enhances levels of cognitive stimulation and physical activity, has therapeutic effects on cognitive, affective and motor function of transgenic HD mice. The present study investigated gene expression changes and behavioral pharmacology in male and female R6/1 transgenic HD mice at an early time-point in HD progression associated with onset of cognitive and affective abnormalities, following EE and exercise (wheel running) interventions. We have demonstrated changes in expression levels of the serotonin (5-HT) receptor Htr1a, Htr1b, Htr2a and Htr2c genes (encoding the 5-HT_1A, 5-HT_1B, 5-HT_2A and 5-HT_2C receptors, respectively) in HD brains at 8 weeks of age, using quantitative real-time PCR. In contrast, expression of the serotonin transporter (SerT, also known as 5-HTT or Slc6a4) was not altered in these brains. Furthermore, we identified region-specific, sex-specific and environmentally regulated (comparing EE, exercise and standard housing conditions) impacts on gene expression of particular 5-HT receptors, as well as SerT. For example, SerT gene expression was upregulated by exercise (wheel running from 6 to 8 weeks of age) in the hippocampus. Interestingly, when EE was introduced from 6 to 8 weeks of age, Htr2a gene expression was upregulated in the cortex, striatum and hippocampus of male mice. EE also rescued the functional activity of 5-HT₂ receptors as observed in the head-twitch test, reflecting sexually dimorphic effects of environmental stimulation. These findings demonstrate that disruption of the serotonergic system occurs early in HD pathogenesis and, together with previous findings, show that the timing and duration of environmental interventions are critical in terms of their ability to modify gene expression. This study is the first to show that EE is able to selectively enhance both gene expression of a neurotransmitter receptor and the functional consequences on behavioral pharmacology, and links this molecular modulation to the therapeutic effects of environmental stimulation in this neurodegenerative disease.

Hypersensitivity to sertraline in the absence of hippocampal 5-HT1AR and 5-HTT gene expression changes following paternal corticosterone treatment

The male germ line is capable of transmitting a legacy of stress exposure to the next generation of offspring. This transgenerational process manifests by altering offspring affective behaviours, cognition and metabolism. Paternal early life trauma causes hippocampal serotonergic dysregulation in male offspring. We previously showed a transgenerational modification to male offspring anxiety-like behaviours by treatment of adult male breeders with corticosterone (CORT) prior to mating. In the present study, we used offspring from our paternal CORT model and characterised offspring serotonergic function by examining their responses to the 5HT_1AR agonist, 8-OH-DPAT, and the selective serotonin reuptake inhibitor, sertraline. We also examined whether post-weaning environmental enrichment, a paradigm well-known to modulate serotonergic signalling in the brain, had the capacity to normalise the anxiety phenotype of male offspring. Finally, we assessed gene expression levels of 5HT_1AR and serotonin transporter in the offspring hippocampus to determine whether deficits in gene transcription contributed to the male-only anxiety phenotype. We report that male and female offspring of CORT-treated fathers are hypersensitive to sertraline but have normal hypothermic responses to 8-OH-DPAT. No deficits in htr1a and sert were found in association with paternal CORT treatment, and environmental enrichment did not rescue the anxiety phenotype of male offspring on the elevated-plus maze. These findings indicate that varying forms of paternal stress exert different effects on offspring brain serotonergic function.

Treadmill exercise delays the onset of non-motor behaviors and striatal pathology in the CAG140 knock-in mouse model of Huntington's disease

Depression, cognitive impairments, and other neuropsychiatric disturbances are common during the prodromal phase of Huntington's disease (HD) well before the onset of classical motor symptoms of this degenerative disorder. The purpose of this study was to examine the potential impact of physical activity in the form of exercise on a motorized treadmill on non-motor behavioral features including depression-like behavior and cognition in the CAG₁₄₀ knock-in (KI) mouse model of HD. The CAG₁₄₀ KI mouse model has a long lifespan compared to other HD rodent models with HD motor deficits emerging after 12months of age and thus provides the opportunity to investigate early life interventions such as exercise on disease progression. Motorized treadmill running was initiated at 4weeks of age (1h per session, 3 times per week) and continued for 6months. Non-motor behaviors were assessed up to 6months of age and included analysis of depression-like behavior (using the tail-suspension and forced-swim tests) and cognition (using the T-maze and object recognition tests). At both 4 and 6months of age, CAG₁₄₀ KI mice displayed significant depression-like behavior in the forced swim and tail suspension tests and cognitive impairment by deficits in reversal relearning in the T-maze test. These deficits were not evident in mice engaged in treadmill running. In addition, exercise restored striatal dopamine D2 receptor expression and dopamine neurotransmitter levels both reduced in sedentary HD mice. Finally, we examined the pattern of striatal expression of mutant huntingtin (mHTT) protein and showed that the number and intensity of immunohistochemical staining patterns of intranuclear aggregates were significantly reduced with exercise. Altogether these findings begin to address the potential impact of lifestyle and early intervention such as exercise on modifying HD progression.

Increased levels of brain serotonin correlated with MMP-9 activity and IL-4 levels resulted in severe experimental autoimmune encephalomyelitis (EAE) in obese mice

The aim of this study was to investigate the role of monoamine neurotransmitters on the severity of experimental autoimmune encephalomyelitis (EAE) in obese mice. EAE was induced in mice with normal diets (ND-EAE) and obese mice with high-fat diets (HFD-EAE) through the immune response to myelin oligodendrocyte glycoprotein (MOG) (35-55). The levels of dopamine (DA), serotonin (5-HT) and their metabolites in different anatomical brain regions were measured by high-performance liquid chromatography. The plasma and tissue NADPH oxidase and matrix metalloproteinases (MMP)-9 activities were analyzed by fluorescence spectrophotometry. The cumulative disease index and disease peaks were significantly higher in HFD-EAE compared with those in ND-EAE. Significantly higher 5-HT levels and lower 5-HT turnovers 5-hydroxyindole acetic acid ((5-HIAA)/5-HT) were found in the brains of HFD-EAE mice compared with those found in the HFD-CON and ND-EAE mice brains. Moreover, increased DA levels were observed in the caudate nucleus of the HFD-EAE mice compared with the control and ND-EAE mice. The NADPH oxidase and MMP-9 activities in the plasma and tissues were significantly higher in both the ND-EAE and HFD-EAE groups than in their respective controls. The cytokine levels in the plasma, tissues, and cultured splenocytes were found to be significantly altered in EAE mice compared with control mice. Moreover, HFD-EAE mice exhibited significantly higher MMP-9 activity and lower IL-4 levels than ND-EAE mice and were significantly correlated with brain 5-HT levels. In conclusion, the increased 5-HT levels in the brain significantly correlated with MMP-9 activity and IL-4 levels play an important role in the exacerbation of disease severity in HFD-EAE mice.

Exercise Regulation of Cognitive Function and Neuroplasticity in the Healthy and Diseased Brain

Regular exercise broadly enhances physical and mental health throughout the lifespan. Animal models have provided us with the tools to gain a better understanding of the underlying biochemical, physiological, and morphological mechanisms through which exercise exerts its beneficial cognitive effects. One brain region in particular, the hippocampus, is especially responsive to exercise. It is critically involved in learning and memory and is one of two regions in the mammalian brain that continues to generate new neurons throughout life. Exercise prevents the decline of the hippocampus from aging and ameliorates many neurodegenerative diseases, in part by increasing adult hippocampal neurogenesis but also by activating a multitude of molecular mechanisms that promote brain health. In this chapter, we first describe some rodent models used to study effects of exercise on the brain. Then we review the rodent work focusing on the mechanisms behind which exercise improves cognition and brain health in both the normal and the diseased brain, with emphasis on the hippocampus.

Viral vector mediated expression of mutant huntingtin in the dorsal raphe produces disease-related neuropathology but not depressive-like behaviors in wildtype mice

Huntington׳s disease (HD) is a neurodegenerative disorder caused by a mutation in the HTT gene (mHTT) encoding the protein huntingtin. An expansion in the gene׳s CAG repeat length renders a misfolded, dysfunctional protein with an abnormally long glutamine (Q) stretch at the N terminus that often incorporates into inclusion bodies and leads to neurodegeneration in many regions of the brain. HD is characterized by motor and cognitive decline as well as mood disorders, with depression being particularly common. Approximately 40% of the HD population suffers from depressive symptoms. Because these symptoms often manifest a decade or more prior to the knowledge that the person is at risk for the disease, a portion of the early depression in HD appears to be a consequence of the pathology arising from expression of the mutant gene. While the depression in HD patients is often treated with serotonin agonists, there is scant experimental evidence that the depression in HD responds well to these serotonin treatments or in a similar manner to how non-HD depression tends to respond. Additionally, at very early sub-threshold depression levels, abnormal changes in several neuronal populations are already detectable in HD patients, suggesting that a variety of brain structures may be involved. Taken together, the serotonin system is a viable candidate. However, at present there is limited evidence of the precise nuclei or circuits that play a role in HD depression. With this in mind, the current study was designed to control for the widespread brain neuropathology that occurs in HD and in transgenic mouse models of HD and focuses specifically on the influence of the midbrain dorsal raphe nucleus (DRN). The DRN provides the majority of the serotonin to the forebrain and exhibits cell loss in non-HD depression. Therefore, we employed a viral vector delivery system to investigate whether the over-expression of mHTT in the DRN׳s ventral sub-nuclei alone is sufficient to produce depressive-like behaviors. Wildtype mice were injected with an adeno-associated virus (AAV2/1) encoding HTT containing either a pathogenic (N171-82Q) or control (N171-16Q) CAG repeat length into the ventral DRN and depressive-like behaviors and motor behaviors were assessed for 12 weeks post-surgery. Quantitative PCR and immunohistochemistry (IHC) verified positive transduction in the ventral aspects of the DRN, including the ventral sub-nucleus (DRv) and interfascicular sub-nucleus (DRif). IHC demonstrated microgliosis in and around the injection site and mHTT-positive inclusions in serotonin-producing neurons and a small percentage of astrocytes in animals injected with N171-82Q compared to controls. Moreover, N171-82Q injected mice showed a 75% reduction in cells that stained positive for the serotonin synthesis enzyme, tryptophan hydroxylase-2 (TPH2) compared to controls (p<0.05). Despite mHTT-mediated pathology in the DRv and DRif, no significant changes in depressive-like behavior were detected. Consequently, we conclude that 12 weeks of N171-82Q expression in the ventral sub-nuclei of the DRN of wildtype mice causes characteristic disease-related cellular neuropathology but is not sufficient to elicit depressive-like behaviors. Ongoing studies are investigating whether a larger injection volume that transfects a larger percentage of the DRN and/or a longer time course of mHTT expression might elicit depressive-like behaviors. Moreover, mHTT expression in other regions of the brain, such as the hippocampal dentate gyrus and/or the frontal cortex might be necessary to elicit HD depression. Together, these results may prove helpful in addressing which therapeutic and/or pharmacological strategies might be most efficacious when treating depressive symptomology in patients suffering from HD.

Effects of chronic stress on the onset and progression of Huntington's disease in transgenic mice

Huntington's disease (HD) is a neurodegenerative disease caused by a tandem repeat mutation encoding an expanded polyglutamine tract. Our previous work showed that memory deficits in HD transgenic mice could be accelerated by increased levels of stress hormone, while memory in WT mice remained unaffected. HD patients experience higher levels of stress compared to the general population and symptoms of HD also include motor, cognitive, psychiatric, sexual and olfactory abnormalities, and an associated decline in activities of daily living. Therefore we investigated the impact of a robust stressor (i.e. restraint) on the onset and progression of a range of behavioral phenotypes in R6/1 transgenic HD mice. Restraint was administered for 1h daily from 6weeks of age and continued until R6/1 mice were clearly motor symptomatic at 14weeks of age. Serum corticosterone levels in both R6/1 and WT littermates were elevated immediately after the last restraint session and weight gain was suppressed in restrained animals throughout the treatment period. Motor coordination and locomotor activity were enhanced by chronic restraint in males, regardless of genotype. However, there was no effect of restraint on motor performances in female animals. At 8weeks of age, olfactory sensitivity was impaired by restraint in R6/1 HD female mice, but not in WT mice. In male R6/1 mice, the olfactory deficit was exacerbated by restraint and olfaction was also impaired in male WT mice. The development of deficits in saccharin preference, Y-maze memory, nest-building and sexually-motivated vocalizations was unaffected by chronic restraint in R6/1 and had little impact on such behavioral performances in WT animals. We provide evidence that chronic stress can negatively modulate specific endophenotypes in HD mice, while the same functions were affected to a lesser extent in WT mice. This vulnerability in HD animals seems to be sex-specific depending on the stress paradigm used. It is hoped that our work will stimulate clinical investigations into the effects of stress on both pre-symptomatic and symptomatic gene-positive members of HD families, and inform the development of new therapeutic approaches.

A Tale of Two Maladies? Pathogenesis of Depression with and without the Huntington's Disease Gene Mutation

Huntington’s disease (HD) is an autosomal dominant disorder caused by a tandem repeat expansion encoding an expanded tract of glutamines in the huntingtin protein. HD is progressive and manifests as psychiatric symptoms (including depression), cognitive deficits (culminating in dementia), and motor abnormalities (including chorea). Having reached the twentieth anniversary of the discovery of the “genetic stutter” which causes HD, we still lack sophisticated insight into why so many HD patients exhibit affective disorders such as depression at very early stages, prior to overt appearance of motor deficits. In this review, we will focus on depression as the major psychiatric manifestation of HD, discuss potential mechanisms of pathogenesis identified from animal models, and compare depression in HD patients with that of the wider gene-negative population. The discovery of depressive-like behaviors as well as cellular and molecular correlates of depression in transgenic HD mice has added strong support to the hypothesis that the HD mutation adds significantly to the genetic load for depression. A key question is whether HD-associated depression differs from that in the general population. Whilst preclinical studies, clinical data, and treatment responses suggest striking similarities, there are also some apparent differences. We discuss various molecular and cellular mechanisms which may contribute to depression in HD, and whether they may generalize to other depressive disorders. The autosomal dominant nature of HD and the existence of models with excellent construct validity provide a unique opportunity to understand the pathogenesis of depression and associated gene-environment interactions. Thus, understanding the pathogenesis of depression in HD may not only facilitate tailored therapeutic approaches for HD sufferers, but may also translate to the clinical depression which devastates the lives of so many people.

Exercise benefits brain function: the monoamine connection

The beneficial effects of exercise on brain function have been demonstrated in animal models and in a growing number of clinical studies on humans. There are multiple mechanisms that account for the brain-enhancing effects of exercise, including neuroinflammation, vascularization, antioxidation, energy adaptation, and regulations on neurotrophic factors and neurotransmitters. Dopamine (DA), noradrenaline (NE), and serotonin (5-HT) are the three major monoamine neurotransmitters that are known to be modulated by exercise. This review focuses on how these three neurotransmitters contribute to exercise affecting brain function and how it can work against neurological disorders.

Long-term consequences of developmental alcohol exposure on brain structure and function: therapeutic benefits of physical activity

Developmental alcohol exposure both early in life and during adolescence can have a devastating impact on normal brain structure and functioning, leading to behavioral and cognitive impairments that persist throughout the lifespan. This review discusses human work as well as animal models used to investigate the effect of alcohol exposure at various time points during development, as well as specific behavioral and neuroanatomical deficits caused by alcohol exposure. Further, cellular and molecular mediators contributing to these alcohol-induced changes are examined, such as neurotrophic factors and apoptotic markers. Next, this review seeks to support the use of aerobic exercise as a potential therapeutic intervention for alcohol-related impairments. To date, few interventions, behavioral or pharmacological, have been proven effective in mitigating some alcohol-related deficits. Exercise is a simple therapy that can be used across species and also across socioeconomic status. It has a profoundly positive influence on many measures of learning and neuroplasticity; in particular, those measures damaged by alcohol exposure. This review discusses current evidence that exercise may mitigate damage caused by developmental alcohol exposure and is a promising therapeutic target for future research and intervention strategies.

Hippocampal neurogenesis, cognitive deficits and affective disorder in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a tandem repeat expansion encoding a polyglutamine tract in the huntingtin protein. HD involves progressive psychiatric, cognitive, and motor symptoms, the selective pathogenesis of which remains to be mechanistically elucidated. There are a range of different brain regions, including the cerebral cortex and striatum, known to be affected in HD, with evidence for hippocampal dysfunction accumulating in recent years. In this review we will focus on hippocampal abnormalities, in particular, deficits of adult neurogenesis. We will discuss potential molecular mechanisms mediating disrupted hippocampal neurogenesis, and how this deficit of cellular plasticity may in turn contribute to specific cognitive and affective symptoms that are prominent in HD. The generation of transgenic animal models of HD has greatly facilitated our understanding of disease mechanisms at molecular, cellular, and systems levels. Transgenic HD mice have been found to show progressive behavioral changes, including affective, cognitive, and motor abnormalities. The discovery, in multiple transgenic lines of HD mice, that adult hippocampal neurogenesis and synaptic plasticity is disrupted, may help explain specific aspects of cognitive and affective dysfunction. Furthermore, these mouse models have provided insight into potential molecular mediators of adult neurogenesis deficits, such as disrupted serotonergic and neurotrophin signaling. Finally, a number of environmental and pharmacological interventions which are known to enhance adult hippocampal neurogenesis have been found to have beneficial affective and cognitive effects in mouse models, suggesting common molecular targets which may have therapeutic utility for HD and related diseases.